What is "blockstack"

Blockstack is the first implementation of a decentralized DNS system on top of the Bitcoin blockchain. It combines DNS functionality with public key infrastructure and is primarily meant to be used by new blockchain applications. Blockstack is being developed by a group of engineers and researchers coming from both industry and academia and is released as open-source. Other systems like Namecoin have implemented decentralized DNS using blockchains earlier by using blockchains other than Bitcoin. Blockstack became operational on the Bitcoin blockchain in September 2015. As of March 2016, Blockstack is the largest non-financial system using the Bitcoin blockchain by transaction volume.